Passivation of stainless steel alloys



PASSIVATION OF STAINLESS STEEL ALLOYS Carlton E. Carrigan, Farmingdale,N.Y., assignor to Fairchild Engine and Airplane Corporation, Deer Park,N.Y., a corporation of Maryland Application December 2, 1957 Serial No.699,912

9 'Claims. (Cl. 148-614) No Drawing.

This invention relates to a method of passivating stainless steel alloysto render them non-reactive to hydrogen peroxide.

While stainless steel alloys by their very nature are considered to beresistant to corrosion and other decomposition in the presence of mostmaterials, it has been Efound that under certain conditions stainlesssteels possess a certain reactivity which is detrimental either to thestainless steel itself or to materials coming in contact with thestainless steel. A prominent example of this is where hydrogen peroxidesolutions are brought in contact with stainless steel. The hydrogenperoxide is relatively unstable and is decomposed, giving rise to oxygenbubbles in the solution. This is detrimental not only for the reasonthat the strength of the hydrogen peroxide solution is lessened, butalso, the bubbles which are formed intenfere in certain operations.Examples of such operations are where the rate of fluidflow of ahydrogenperoxide solution is measured in a flowmeter.

The flowmeter conventionally employed for measuring the rate of fluidflow of a hydrogen peroxide solution consists of a transparent tubemounted vertically, and gradually tapering from a small inside diameterat the bottom of the tube to a larger inside diameter at the top. Thefluid flows vertically upward through the tube, causing a float to risein the tube in accordance with the rate of flow of the fluid. Thedistance traveled by the float is directly proportional to the rate offlow and is determined by a scale on the tube.

The floats and other parts conventionally employed in flowmeters used inmeasuring the rate of flow of hydrogen peroxide solutions are fabricatedof a stainless steel alloy. The body of the flowmeter may also be of thesame material. If the hydrogen peroxide flowing past the float or otherstainless steel parts is decomposed, gas bubbles will adhere to thefloat causing it to read incorrectly. In addition, any gas bubblesformed by decomposition of the hydrogen peroxide caused by contact withstainless steel prior to its reaching the flowmeter will also cause asource of reading error. In addition to the errors caused by the gasbubbles formed, the loss of oxygen represented thereby significantlydecreases the strength of the hydrogen peroxide solution. These factorsare of utmost importance in the field of hydrogen peroxide power plantsand it thus becomes desirable to passivate the stainless steel partsemployed in such power plants to render them inert to hydrogen peroxide.

Several methods are currently employed in passivating stainless steelalloys. In general, they comprise the steps of immersing the stainlesssteel in a pickling solution and then immersing it in a solution ofhydrogen peroxide until all bubblingceases. While a process of thisnature is of some benefit, it is found that the passivation thusobtained is not of satisfactory duration, and that after a short periodof time the stainless steel again becomes reactive to hydrogen peroxide.With some stainless steels, such as Type 303, the prior art methods arecompletely unsatisfactory.

It has now been found that by heating the stainless steel alloy in anoxidizing atmosphere after the pickling step, but prior to thecontacting thereof with hydrogen peroxide, a more complete passivationof the alloy is obtained which will last for a much longer period oftime than the passivation obtained according to prior art practices.This step is of particular benefit with stainless steel alloys having arelatively low content of chromium and nickel, such as that commonlydesignated Type 303 according to American Iron and Steel Institute typenumbers established in June 1939. Various other stainless steels,however, are also benefited by treating them according to the process ofthis invention. The passivation obtained with the Type 303 stainlesssteel according to the method of this invention may possibly be of apermanent nature, no failures having been obtained at this time.

As an indication of the types of stainless steel which are benefitedaccording to this invention, reference is made to the Chemical EngineersHandbook by John H. Perry, The McGraW-Hill Book Company, Inc., 1941,pages 21192l21 and 2141-2142. The stainless steels there listed areidentified by type numbers assigned by the American Iron and SteelInstitute. Type 303 stainless steel contains 0.20% maximum carboncontent, 17.520.0% chromium, 8.004000% nickel, sulphur or selenium 0.07%minimum or molybdenum 0.60% maximum.

The step of heating the stainless steel alloy in an oxidizing atmosphereis very important to the process of the present invention. The oxidizingatmosphere which is preferably employed is air, but any other suitableoxidizing atmosphere may be employed.

It has been found that heating the alloy to a temperature of about 750F. for about two hours provides the desired eifect where an objecthaving a size similar to that of the float used in the flowmeter isbeing treated. The temperature may vary somewhat from. the 750 F. level.Temperatures within the range of about 700 F. to about 800 3?. have beenfound to provide good results. In addition, the heating time employed inthe process of this invention may vary, depending upon the size of theobject being heated. Objects which are smaller than the float mayrequire somewhat less time for a complete passivation, while a large anddense stainless steel object may require considerably more time than twohours. In any event, the time and temperature required for a givenstainless steel article may be readily determined by simpleexperimentation. In general, heating times from thirty minutes. toseveral hours will be employed.

The following example further illustrates the process of this invention.

Example 1 A flowmeter float made from Type 3-03 stainless steel alloywas immersed in a solution of 20% caustic soda for thirty minutes atroom temperature. It was then rinsed and immersed. in a solution of 50%nitric acid (for twenty-four hours at room temperature.

The float was then pickled in a- 10% chromic acid solution at roomtemperature for about one hour, rinsed, and then pickled in a sodiumpicrate solution at its boiling point for one hour. The sodium picratesolution contained 245 grams of sodium hydroxide and 20 grams of picricacid in 735 ml. of water.

After the pickling process, the float was rinsed, heated to 750 F. fortwo hours in air, air cooled, and; immersed in a solution of 30%hydrogen peroxide until all reaction ceased.

If desired, the float may be immersed in a hydrogen peroxide solutionafter pickling and before the heat treatment step to determine thepassivity of the alloy at that stage. This is not an essential processstep, however.

After passivating the stainless steel float in this manner, it wasemployed in a flowmeter to determine the rate of flow of a hydrogenperoxide solution. Each of the other stainless steel parts of theflowmeter had been passivated in the same manner. No interfering bubbleswere formed on the float or in the hydrogen peroxide solution itself.This made accurate determination of the rate offlow possible andprevented any deterioration in the strength of the hydrogen peroxidesolution passing through the flowmeter. Furthermore, the passivationobtained in this manner is of a much more permanent nature than thepassivation obtained by prior art techniques.

It is understood that the passivation process referred to throughout thespecification is applied to and is effective only on the surface of thestainless steel alloy. If the alloy is machined after passivation, theentire process must be repeated in order to provide a properlypassivated alloy.

While the process of this invention has been described with reference toa particular type of stainless steel, and to a small float and otherparts of a flowmeter, it should be understood that the process isapplicable to other types of stainless steel, and in different forms.

I claim:

1. In the process of passivating a stainless steel alloy including thesteps of immersing the alloy in a pickling bath and thereafter exposingthe alloy to a solution of hydrogen peroxide until all reaction ceases,the improvement comprising the inclusion of the step of heating thealloy in an oxidizing atmosphere after exposure of the alloy to thepickling bath and before reacting the alloy with hydrogen peroxide.

2. In the process of passivating a stainless steel alloy including thesteps of immersing the alloy in a pickling bath and thereafter exposingthe alloy to a solution of hydrogen peroxide until all reaction ceases,the improvement comprising the inclusion of the step of heating thealloy in an oxidizing atmosphere at a temperature of from about 700 F.to about 800 F. after exposure of the alloy to the pickling bath andbefore reacting the alloy with hydrogen peroxide.

3. In the process of passivating a stainless steel alloy including thesteps of immersing the alloy in a pickling bath and thereafterexposingthe alloy to a solution of hydrogen peroxide until all reactionceases, the improvement comprising the inclusion of the step of heatingthe alloy in an oxidizing atmosphere at a temperature of from about 700F. to about 800 F. for from one-half hour to several hours, the time ofheating depending upon the mass of the alloy and the heating step takingplace after exposure of the alloy to the pickling bath andbefore'reacting'the alloy with hydrogen peroxide.

4. A method of passivating a stainless steel alloy which comprisesimmersing the'alloy in a pickling solution, rinsing, heating the alloyat a temperature of from 2,890,974 a p i 7 6. A method of passivating astainless steel alloy to render it substantially non-reactive tohydrogen peroxide solutions comprising immersing the alloy in a 20%caustic soda solution at room temperature for about thirty -minutes,rinsing the alloy, immersing the alloy in a solution of 50% nitric acidat room temperature for about twenty-four hours, rinsing the alloy,immersing the alloy in a 10% chromic acid solution at room temperaturefor one hour, rinsing, immersing the alloy in a freshly prepared sodiumpicrate solution of about 2% concentration at the boiling point thereoffor one hour, rinsing the alloy, heating the alloy to about 750 F. forabout two hours in air, air cooling and immersing the alloy about 700 F.to about 800 F. in an oxidizing atmosphere, cooling the alloy andreacting the alloy with hydrogen peroxide until all reaction ceases.

' 5. A method of passivating a stainless steel alloy which comprisesimmersing the alloy in a strong alkali, rinsing, immersing the alloy ina strong acid solution, rinsing, immersing the alloy in a solution ofsodium picrate, rinsing, heating the stainless steel in an oxidizingatmosphere at a temperature of from about 700 F. to about 800 F. forfrom one-half hour to several hours depending upon the mass of thealloy, cooling in air, and immersing the alloy in a solution of hydrogenperoxide until reaction ceases.

in a 30% solution of hydrogen peroxide until all reaction ceases.

7. A method of passivating AISI Type 303 stainless steel alloy having nomore than 0.20% carbon, from 17.5 to 20% chromium, from 8 to 10% nickel,a minimum of 0.07% sulphur and molybdenum, at a maximum level: of 0.60%to render it substantially non-reactive to hydrogen peroxide solutionscomprising immersing the alloy in a 20% caustic soda solution at roomtemperature for about thirty minutes, rinsing the alloy, immersing thealloy in a solution of 50% nitric acid at room temperature for abouttwenty-four hours, rinsing the alloy, immersing the alloy in 10% chromicacid solution at room temperature for one hour, rinsing the alloy,immersing it in a freshly prepared sodium picrate solution containingabout 2% of picric acid and an excess of sodium hydroxide at the boilingpoint of said solution for one hour, rinsing the alloy, heating thealloy to about 750 F. for about two hours in an oxidizing atmosphere,cooling, and immersing the alloy in a 30% solution of hydrogen peroxideuntil all reaction ceases.

8. A method of passivating the surface of AISI Type 303 stainless steelalloy including no more than 0.15% carbon, from 17.5 to 19% chrominum,from 8 to 10% nickel, at least 0.15% sulphur, and molybdenum at amaximum level of 0.60%, to render it substantially nonreactive tohydrogen peroxide solutions comprising immersing the alloy in a 20%caustic soda solution at room temperature for about thirty minutes,rinsing the alloy, immersing the alloy in a 50% solution of nitric acidat room temperature for about twenty-four hours, rinsing the alloy,immersing the alloy in a 10% chromic acid solution at room temperaturefor about one hour, rinsing, immersing the alloy in a freshly preparedsodium picrate solution containing about 2% by weight of picric acid andabout 25% by weight of sodium hydroxide at the boiling point thereof forabout one hour, rinsing the alloy, heating the alloy to about 750 F. forabouttwo hours in an oxidizing atmosphere, cooling, and immersing thealloy in a concentrated solution of hydrogen peroxide until all reactionceases.

9. In the process of passivating a stainless steel alloy including thesteps of immersing the alloy in acid and alkali pickling solutions toremove non-metallic impurities and thereafter exposing the alloy to asolution of hydro;

References Cited in the file of this patent UNITED STATES PATENTS Wulfiet al. Sept. 12, 1939 Thurber June 14, 1949

4. A METHOD OF PASSIVATING A STAINLESS STEEL ALLOY WHICH COMPRISESIMMERSING THE ALLOY IN A PICKLING SOLUTION, RINSING, HEATING THE ALLOYAT A TEMPERATURE OF FROM ABOUT 700*F. TO ABOUT 800*F. IN AN OXIDIZINGATMOSPHERE, COOLING THE ALLOY AND REACTING THE ALLOY WITH HYDROGENPEROXIDE UNTIL ALL REACTION CEASES.